In an ion trap device, ions are trapped in a three-dimensional quadrupole electric field generated basically by combining an RF electric field and a DC electric field. One type of ion trap device is constructed with electrodes whose inner surfaces are shaped hyperboloid-of-revolution so that a rather large ion trapping space is created in the space surrounded by the electrodes. Another type of ion trap device is constructed with cylindrical and disc electrodes (Cylindrical Ion Trap) in which an ion trapping space is created around the center of the space surrounded by the electrodes. In these constructions, the electrodes are composed of a ring electrode and two end cap electrodes placed at both ends of the ring electrodes, wherein the RF voltage for trapping ion is normally applied to the ring electrode. In either electrode construction, the mass to charge ratio (m/e) of an ion determines whether the ion is trapped in the trapping space in a stable manner, or its movement becomes unstable and it collides with the electrodes or is ejected from an opening of the electrodes. The theory of an ion trapping method is explained in, for example, R. E. March and R. J. Hughes, “Quadrupole Storage Mass Spectrometry”, John Wiley & Sons, 1989, pp. 31–110.
The RF voltage applied to the ring electrode is generated as follows. A coil is connected to the ring electrode, and an LC resonant circuit is formed by the inductance of the coil, the capacitance inherently formed between the ring electrode and the end cap electrodes, and the capacitance included in and associated with all the other circuit elements. To the LC resonant circuit, an RF driving circuit is connected directly or indirectly with a transformer coupling. Owing to the configuration, the Q value of the resonant circuit is rather high, so that an RF voltage with a large amplitude (which will be referred to as “RF high voltage”) can be applied to the ring electrode with a small RF driving voltage. Usually, the resonant frequency of the LC resonant circuit is adjusted to the frequency of the RF driving circuit with a tuning circuit using a variable capacitor.
There are some problems in the above resonant circuit. As the temperature rises, the coil expands and its inductance changes, or the capacitance of the variable capacitor changes, which leads to a deviation of the resonant frequency of the resonant circuit from the frequency of the RF driving circuit. Normally a high voltage switch is provided to the ring electrode, and the capacitance of the high voltage switch changes as the RF high voltage is changed. This also breaks the resonance condition of the resonant circuit.
Usually, a feedback control is performed to maintain the amplitude of the RF high voltage constant by adjusting the output voltage of the RF driving circuit, so that the amplitude of the RF high voltage does not change even if the resonant frequency of the resonant circuit deviates from the frequency of the RF driving circuit. But the phase of the RF high voltage deviates from that of the output of the RF driving circuit. In the ion trap device, various processes are performed relating to or using the phase of the RF high voltage applied to the ring electrode. The selection and/or dissociation of ions in the ion trap is one of such processes. In such processes, normally, the phase information is derived from the RF driving circuit, and various timings are determined based on this phase information. When, therefore, there is a deviation between the phases of the output of the RF driving circuit and the RF high voltage, the accuracy of the process is impaired, or a proper process is impossible.
For example, when ions having various mass to charge ratios are analyzed, ions are successively ejected from the ion trap according to their mass to charge ratios while the RF high voltage applied to the ion trap is scanned. In that case, the timing when ions are ejected from an ion trap is related to the phase of the RF high voltage, and the positions of the peaks of a mass spectrum shift if there is a deviation in the phases of the RF high voltage and the RF driving circuit.
Another example is as follows. When ions are ejected from an ion trap to a TOFMS to mass analyze the ions, the kinetic energy of the ions when ejected and their ejecting direction are related to the phase of the RF high voltage when the ions are ejected. If there is a deviation in the phases, similarly, the peak positions of a mass spectrum shift.
Such a problem can be avoided, in principle, by deriving the phase signal not from the RF driving voltage, but directly from the RF high voltage which is amplified by resonance, and determine various timings based on the phase of the RF high voltage. But in practice it is very difficult to derive exact phases from the RF high voltage whose amplitude is always changing depending on respective stage of a mass analysis, and it will be very expensive and impractical to design such a monitoring circuit. Moreover it is impossible to incorporate such a function to a device already in use.
In the Japanese Unexamined Patent Publication No. 2004-152658 (which corresponds to the U.S. Pat. No. 6,870,159, and is hereinafter referred to as “Reference 1”), the following ion trap device is disclosed. A driving voltage generated by an RF driving circuit is amplified by a resonant circuit, and the amplified RF voltage is applied to at least one of electrodes constituting an ion trap. The resonant circuit includes a tuning circuit to change the resonant frequency of the resonant circuit, and the resonant frequency of the resonant circuit is controlled to deviate from the frequency of the RF driving voltage. Owing to the control, the resonant frequency of the resonant circuit is intentionally shifted from the frequency of the RF driving voltage. This decreases the influence of the change in the resonant frequency when the RF high voltage is changed to the difference in the phases of the RF driving voltage and the RF high voltage. Thus the shift of the peak positions in a mass spectrum is prevented, and the accuracy and sensitivity of mass analyses are enhanced because various qualities of the mass spectrometer having their base to the phases of the RF high voltage are prevented from deteriorating.
If the cause of the difference in the phases of the RF driving voltage of the RF driving circuit and the RF high voltage depends on the amplitude of the RF high voltage, the shifting direction should be properly controlled, otherwise the oscillation of the resonant circuit cannot be stable. For example, when a semiconductor element is connected to the electrode of the ion trap to which the RF high voltage is applied, the effective capacitance of the semiconductor element increases as the amplitude of the RF high voltage increases, and the resonant frequency of the resonant circuit decreases. Here it is supposed that the resonant frequency is increased from the frequency of the RF driving voltage by decreasing the capacitance of the tuning circuit. If the amplitude of the RF high voltage is increased, the capacitance of the semiconductor element increases, and the resonant frequency comes closer to the frequency of the RF driving voltage. This increases the gain of the resonant circuit, and constitutes a positive feedback that deteriorates the stability of the resonant circuit. Thus, when a semiconductor element is connected to such an electrode, it is necessary to adequately control the shifting direction of the resonant frequency so that the resonant frequency decreases. This can be done by increasing the capacitance using a tuning circuit, for example increasing the value of a variable capacitor. Generally speaking, if the resonant frequency changes to a certain direction when the amplitude of the RF high voltage increases, it is preferable to adjust the tuning circuit so that the resonant frequency is shifted to the same direction. This stabilizes the oscillation, and assures the above effects of enhancing the accuracy and sensitivity of mass analyses.